In the current 32 bit system processor architecture, an application may invoke the processor to execute code written for a 16 bit operating mode in a 16 bit operating mode. For instance, many devices include device code in their firmware, such as service routines and interrupts that are intended to be executed in a 16 bit environment. This device code may need to be called during operation in the 32 bit mode to control the device, such as change video settings in a video processor, etc.
The Intel Corporation 386 and above 32 bit processor architectures include a Virtual-8086 (V86) mode that emulates a 16 bit environment in which 16 bit instructions, such as device code, may be executed. The processor may switch between V86 mode and protected mode. The processors enters V86 mode from protected mode to execute 16 bit device code, such as an 8086 program, then leaves V86 mode and enters protected mode to continue executing a native 32 bit program, e.g., 80386 program. Further details of V86 mode are described in the publication “Intel Architecture Software Developer's Manual, Volume 3: System Programming” (Copyright Intel, 1999).
For the 64 bit architecture, emulators have been developed because V86 mode is not supported in the 64 bit architecture. Emulators allow execution of instructions intended for 16 bit real mode by emulating the real mode 16 bit device code as 64 bit instructions. Another option is to allow the 64 bit operating system transition to 32-bit protected mode in which the 16 bit mode execution can occur, but this entails having separate interrupt handlers and is costly from a systems and support perspective.